1. Field of the Invention
This invention relates to microelectromechanical devices, and more particularly, to a microelectromechanical device in which a cantilever is electrostatically pulled away from a conductive pad.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Microelectromechanical devices, or devices made using microelectromechanical systems (MEMS) technology, are of interest in part because of their potential for allowing integration of high-quality devices with circuits formed using integrated circuit (IC) technology. For example, MEMS switches may exhibit lower losses and a higher ratio of off-impedance to on-impedance as compared to transistor switches formed from conventional IC technology. However, a persistent problem with implementation of MEMS switches has been the high voltage required (often about 40V or higher) to actuate the switches, as compared to typical IC operating voltages (about 5V or lower).
These relatively high actuation voltages of MEMS switches are caused at least in part by a tradeoff between the closing and opening effectiveness of a given switch design. In the case of a cantilever switch, for example, approaches to lowering the actuation voltage of the switch include reducing the stiffness of the cantilever beam and reducing the gap between the beam and the underlying conductive pad. Unfortunately, these design changes typically have the effect of making opening of the switch more difficult. MEMS cantilever switch designs generally use an applied voltage to close the switch, and rely on the spring force in the beam to open the switch when the applied voltage is removed. In opening the switch, the spring force, or restoring force, of the beam must typically counteract what is often called xe2x80x9cstictionxe2x80x9d. Stiction refers to various forces tending to make two surfaces stick together, such as van der Waals forces, surface tension caused by moisture between the surfaces, and/or bonding between the surfaces (e.g., through metallic diffusion). In general, modifications to a switch which act to lower the closing voltage also tend to make the switch harder to open, such that efforts to form a switch with a lowered closing voltage can result in a switch which may not open reliably (or at all).
Another problem with MEMS devices is that they tend to deform due to residual stresses contained within the devices. More specifically, the residual stresses within a MEMS switch may cause a beam within the device to curl either away from its underlying contact structures or toward the contact structures. In the event that the beam curls down and closes a contact prematurely, the switch may become inoperable because significant electrostatic repulsion between the gate and the beam cannot be established. In this manner, the switch may not be opened by removing an applied voltage as described above. Residual stresses typically arise when a MEMS device has layers of differing properties. For example, the device may include layers of differing materials. Alternatively or in addition, the properties of the layers may change if deposition conditions change as the layers are formed. As such, the variation of materials within conventional MEMS devices may be limited. In addition, fabrication steps may be tightly controlled such that changes in layer properties do not occur.
It would therefore be desirable to develop a MEMS device which relaxes the constraints imposed by the above-described tradeoff between opening and closing effectiveness and the presence of residual stresses within the device.
The problems outlined above may be in large part addressed by a device adapted to electrostatically pull a cantilever away from a conductive pad and a method for fabricating such a device. In particular, a microelectromechanical device is provided which includes a fulcrum contact structure interposed between two electrodes spaced under a cantilever. The device further includes a conductive pad arranged under the distal end of the cantilever and laterally adjacent to one of the electrodes. Such a device may be adapted to initially bring the cantilever in contact with the conductive pad by an application of a closing voltage to one of the electrodes. The device may be further adapted to deflect the cantilever away from the conductive pad upon an application of voltage to the other of the electrodes such that the cantilever contacts the fulcrum contact structure. In another embodiment, the device may be adapted to deflect the cantilever away from the conductive pad upon a release of the closing voltage after the application of the voltage to the other electrode. In yet another embodiment, the device may be adapted to deflect the cantilever away from the conductive pad upon an increase of the voltage applied to the other electrode after a release of the closing voltage.
In some embodiments, the cantilever may include residual forces with which to bring the cantilever in contact with the conductive pad. In such an embodiment, the application of voltage to one or both of the electrodes may pull the cantilever in contact with the fulcrum contact structure. In this manner, the device may serve as a functional switch since contact at the fulcrum structure may be made and/or released by actuating either one or both of the gate structures. In addition, the application of a voltage to an electrode interposed between the fulcrum contact structure and a support structure of the cantilever, in such an embodiment, may be sufficient to pull the cantilever apart from the conductive pad. In an alternative embodiment, the residual stresses within the cantilever may cause the beam to curl away from the conductive pad. In such an embodiment, the device may be adapted to pull the cantilever away from the fulcrum contact structure. In particular, the application of a voltage to an electrode arranged laterally adjacent to the contact pad, in such an embodiment, may be sufficient to pull the cantilever apart from the fulcrum contact structure. In addition, the device may be adapted to bring the cantilever in contact with both the conductive pad and the fulcrum contact structure.
In an embodiment, a microelectromechanical device as described above may include first and second electrodes spaced under a cantilever. In addition, the device may include a fulcrum contact structure interposed between the first and second electrodes and a conductive pad arranged under a distal end of the cantilever and laterally adjacent to the second electrode. In some embodiments, the conductive pad may be interposed between the fulcrum contact structure and the second electrode. Alternatively, the second electrode may be interposed between the fulcrum contact structure and the conductive pad. In addition, the conductive pad and/or fulcrum contact structure may include multiple sections spaced apart from each other.
The cantilever may be supported by a support structure at the end opposite the distal end of the cantilever. In some embodiments, the support structure may include an electrical terminal. The cantilever may further include an insulating element interposed between the supported end and the distal end of the cantilever. In addition or alternatively, the cantilever may have a dimpled portion above at least one of the fulcrum contact structure and conductive pad. On the other hand, the cantilever may be substantially uniform. In addition or alternatively, at least one of the fulcrum contact structure and the conductive pad may include a raised section arranged upon its respective surface. Regardless of the configuration, the spacing between the fulcrum contact structure and its overlying respective portion of the cantilever is preferably smaller than the spacing between the first and second electrodes and their overlying respective portion of the cantilever when the cantilever is not in contact with the conductive pad. As such, the cantilever may include a dimpled portion above at least the fulcrum contact structure. In addition or alternatively, an upper surface of the fulcrum contact structure may be above an upper surface of the conductive pad. On the contrary, an upper surface of the fulcrum contact structure may be below an upper surface of the conductive pad.
As stated above, a microelectromechanical device adapted to electrostatically pull a cantilever apart from a conductive pad is provided. Such a device may include first and second electrodes spaced under the cantilever and the conductive pad arranged laterally adjacent to the second electrode. In addition, the device may include a fulcrum contact structure interposed between the first and second electrodes. In one embodiment, the fulcrum contact structure may be arranged under the center point of the cantilever. Alternatively, the fulcrum contact structure may be arranged either closer to the distal end or supported end of the cantilever. In some cases, the fulcrum contact structure may include conductive material.
The device may be adapted to pull the cantilever apart from the conductive pad upon application of an activation voltage to the first electrode such that the cantilever contacts the fulcrum contact structure. in some embodiments, the device may be further adapted to initially bring the cantilever in contact with the conductive pad by an application of a closing voltage to the second electrode. In such an embodiment, the device may be adapted to pull the cantilever away from the conductive pad upon an application of an activation voltage to the first electrode. In some cases, the device may be adapted to pull the cantilever apart from the conductive pad upon a release of the closing voltage applied to the second electrode in addition to the application of the activation voltage to the first electrode. The combination of the applying the actuation voltage and releasing the closing voltage may be conducted in series or simultaneously. In an alternative embodiment, the device may be adapted to pull the cantilever apart from the conductive pad upon an increase of the activation voltage after the release of the closing voltage.
In some embodiments, the cantilever may contain residual forces, which may be adapted to initially bring the cantilever into contact with the conductive pad. An application of the closing voltage to the second electrode in such an embodiment may bring the cantilever in contact with the fulcrum contact structure. In some cases, the application of the closing voltage may be applied in combination with an application of the actuation voltage to the first electrode. Such applications of voltages may be conducted simultaneously or in series. Alternatively, contact to the fulcrum contact structure may be made only by the application of the actuation voltage to the first electrode. In either embodiment, the device may serve as a functional switch since contact may be made and/or released at the fulcrum contact structure by actuating either one or both gates. In addition, the application of the actuation voltage to the first electrode may pull the cantilever away from the conductive pad.
In an alternative embodiment, the residual forces contained within the cantilever may be adapted to curl the cantilever away from the conductive pad. In such an embodiment, the device may be adapted to initially bring the cantilever in contact with the fulcrum contact structure upon an application of an actuation voltage to the first electrode. Furthermore, the device may be adapted to initially bring the cantilever in contact with the fulcrum contact structure upon an application of an actuation voltage to the first electrode and a simultaneous application of a closing voltage to the second electrode. Additionally the device may be adapted to initially bring the cantilever in contact with both the fulcrum contact structure and the conductive pad with a simultaneous application of voltages to the first and second electrodes. In embodiments such as these, the device may be adapted to electrostatically pull the cantilever away from the fulcrum contact structure. In particular, the device may be adapted to pull the cantilever apart from the fulcrum contact structure upon an application of a closing voltage to the second electrode such that the cantilever contacts the conductive pad. In some embodiments, the device may be adapted to pull the cantilever apart from the fulcrum contact structure upon an application of a closing voltage to the second electrode, such that the cantilever contacts the conductive pad, and a release of the actuation voltage applied to the first electrode. The combination of applying the closing voltage and releasing the actuation voltage may be conducted serially or simultaneously. In an alternative embodiment, the device may be adapted to pull the cantilever apart from the fulcrum contact structure upon an increase of the closing voltage after the release of the actuation voltage.
A method for fabricating the microelectromechanical device as described above is also contemplated herein. In particular, the method may include forming a first electrode and a second electrode upon a substrate. In some cases, the width of the first electrode may be greater than the width of the second electrode. The method may continue by patterning a fulcrum contact structure between the first and second electrodes and patterning a conductive pad laterally adjacent to the second electrode. In some embodiments, the conductive pad may be interposed between the second electrode and the fulcrum contact structure. Alternatively, the second electrode may be interposed between the conductive pad and fulcrum contact structure. In some cases, the conductive pad and/or fulcrum contact structure may include raised sections upon their respective surfaces. In such an embodiment, patterning the conductive pad and/or fulcrum contact structure may include patterning a base structure of the respective structure and subsequently patterning a raised section from the upper portion of the base structure.
The method may further include forming a cantilever spaced above the first and second electrodes, conductive pad, and fulcrum contact structure. Forming the cantilever may include forming a sacrificial layer upon the first and second electrodes, the conductive pad, the fulcrum contact structure, and exposed portions of the substrate. The formation of such a sacrificial layer may include depositing the sacrificial layer upon the first and second electrodes, the conductive pad, the fulcrum contact structure, and exposed portions of the substrate. Recesses may then be etched within the deposited sacrificial layer above at least one of the conductive pad and fulcrum contact structure. Alternatively, the recesses may be formed by pattern depositing the sacrificial layer in multiple steps. The method may further include depositing a beam layer upon the sacrificial layer. Finally, the sacrificial layer may be removed such that the cantilever is spaced above the electrodes, conductive pad, and fulcrum contact structure. In addition, the method may include forming a support structure laterally adjacent to the first electrode prior to forming the cantilever. In some embodiments, the support structure may include an electrical terminal.
There may be several advantages to forming a device that is adapted to electrostatically pull a cantilever away from a conductive pad. For example, such a device may overcome the opening difficulties associated with surface tension issues, such as stiction. As such, a more flexible beam may be employed within the device. Consequently, the device may operate at lower actuating voltages, thereby making implementation with integrated circuits more feasible. In addition, the functionality of the device as described herein is not restricted by residual stresses contained within the device since a repulsive electrostatic force between the gate and beam is not required to exist in order to deflect the cantilever from the conductive pad. In other words, the device as described herein may deflect a cantilever that has bent down in contact with the conductive pad without the influence of an electrostatic force.